1. Field of the Invention
This invention relates generally to cadmium negative electrodes for use in electrochemical cells and the process of manufacture thereof and, more particularly, to flexible cadmium electrodes which are capable of being wound in a jelly-roll configuration for use in rechargeable nickel-cadmium alkaline cells. Specifically, the present invention relates to an improved cadmium electrode and cell having enhanced anti-agglomeration characteristics.
2. Description of the Prior Art
The typical known cadmium electrode consists of a mixture of cadmium oxide, cadmium hydroxide, cadmium metal, a minor amount of nickel hydroxide to retard cadmium agglomeration and capacity fade, and a binder to provide integrity to the mixture and secure attachment to an electrically conductive substrate. In preparing pasted cadmium electrode plates, it has been common practice to use a water soluble or water dispersible resin as the binder for the electrochemically active material. However, use of water to blend the electrochemically active ingredients with the binder has several drawbacks. A principal drawback in that the water reacts with cadmium oxide to form the corresponding hydroxide which is of lower density. The result is a reduction in capacity of the electrode as well as pollution of process water. The hydroxide form presents the further drawback in that it reacts with air on stand to form the undesirable carbonate species. Water in the presence of air also promotes conversion of the cadmium metal to cadmium hydroxide.
Perhaps the best prior art non-sintered cadmium electrodes have been made using polytetrafluoroethylene (PTFE) dispersible resin in water. The PTFE binder holds the electrode mass together by a series of fibrillate fibers throughout the active mass and serves to enhance recombination of oxygen at the negative cadmium electrode on charge and overcharge. It is also known to catalyze this oxygen recombination reaction by impregnating the active mass of the cadmium electrode with sub-microscopic silver particles, as taught in U.S. Pat. No. 3,877,985 (Rampel). Typically, this type of Teflon bonded electrode has been prepared according to the teachings of U.S. Pat. No. 3,954,501 (Rampel), the disclosure of which is herein incorporated by reference. In this process cadmium oxide is hydrated to typically about 70 percent. Nickel hydroxide up to about 2.0 weight percent is also added to retard cadmium agglomeration (densification) as taught in U.S. Pat. No. 3,870,562 (Catherino). Nevertheless, cycle life of sealed nickel-cadmium cells incorporating these cadmium electrodes is limited primarily by the agglomeration phenomenon to about 250 cycles in certain applications. The agglomeration causes a gradual reduction of the active electrode surface area due to densification of the cadmium metal (charge state) as well as to change in the size of cadmium hydroxide crystals. Thus, the effective current density gradually increases, and the capacity of the cell measured in ampere hours (Ah) deliverable to a fixed cut-off voltage decreases. For applications requiring long cycle life, the usefulness of this known cell is limited accordingly.
Japanese patent publication no. 57-174864 assigned to Matsushita recognizes the disadvantages of transformation of cadmium oxide to cadmium hydroxide when employing water soluble pastes. This patent publication discloses the use of poly (vinyl) alcohol as a binder dispersed in ethylene glycol or propylene glycol non-aqueous solvents. The strength and conductivity of this electrode are improved by adding resin or metal fibers to the paste. However, these fibers are acknowledged to increase the difficulty of extruding or slurry coating the electrode substrate as it is pulled through a slit. The fibers used include acrylonitrile-vinyl chloride copolymer, polyamide and polyvinylchloride as resin fibers, and steel, nickel, and nickel plated resin fiber as metal fibers. The capacity of the resultant electrode is disclosed as 75 mAh/cm.sup.2.
Japanese patent publication no. 52-31348 assigned to Furukawa discloses a method for preparing a cadmium electrode including the steps of filling and then drying a paste formed by mixing a powdered cadmium active material, a binder solution, and a lower melting point nylon powder composed of a copolymer of nylon 6, nylon 66 and nylon 12, in a substrate, and heating the paste at a temperature exceeding the melting point of the lower melting point nylon whereupon the nylon powder is melted to bind the active material powders. It is believed this method will fail to produce an electrode with satisfactory dispersion of the nylon, and that the in situ melted nylon will mask the activity of the cadmium. Most importantly this reference does not disclose use of a material which will retard cadmium agglomeration during electrode cycling.
The state of the prior art of cadmium electrodes is also represented by the teaching of Japanese patent publication no. 54-106830 (assigned to Furukawa and reported in CA 92:8884m). According to that reference a paste consisting of cadmium oxide or cadmium hydroxide and a binder is applied on to a porous sheet coated with thermally fusible nylon grains. The pasted sheet is then dried and conversion treated to prepare cadmium anodes. Nylon grains of 0.2-0.4 mm are disclosed as being blown onto a nickel-plated iron sheet, and a paste consisting of cadmium hydroxide, nickel powder, short fibers, poly (vinyl) alcohol, carboxymethyl cellulose, and water is applied to the nylon-coated sheet with a reported reduction in the percentage of defective electrodes produced. It is believed the nylon serves as a binder to promote cohesion between the active material and substrate.
Yet another effort to address the problem of binding as well as agglomeration in cadmium electrode structures includes a copending U.S. application Ser. No. 07/382,816, filed Jul. 19, 1989, now U.S. Pat. No. 5,064,735 entitled "Cadmium Electrode and Process for its Production", which application has a partially common inventorship as well as a common assignee with the subject application, the contents of which are specifically incorporated herein by reference. In this disclosure, a cadmium electrode employs an anti-agglomerating material which also serves as a binder. This material is in a form of a micro-porous interconnecting polymeric network. The polymeric network is intimately admixed with the active cathode material in which it functions as an electrolyte wick to maintain the electrolyte at the active porous surfaces of the electrode retained in the polymeric network, during charge and discharge of the electrode.
While the above referenced polymeric network considerably enhances the anti-agglomeration characteristics of the disclosed cathode, there is still a desire for a cadmium electrode structure which is simple to construct and which provides significantly enhanced anti-agglomeration characteristics resulting in enhanced cell life and capacity.